I've never had this happen when my wii remote or television remained in the same location after calibration.... Make sure that either is not moved after calibration (I'm sure you've already thought of this, but I thought I would mention it)
It's definitely not the trial version of smoothboard

This trial version functions the same way as the registered version.
I'm assuming the height of your interactive surface is just over 20 inches, so I would try moving the wii remote closer to the screen (to just over 40 inches). The wii remote should be at a distance of twice the height of the screen from the interactive surface.
I'm also wondering if you are using an infrared pen with the Vishay TSAL 6400 LED...
If your television does not reflect the infrared light well towards the wii remote, this may also be a contributing factor. However, moving the wii remote closer may fix this problem.
Hope this helps!